gn: Support build directories outside the source tree.

tools/gn/function_read_file.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/files/file_util.h"
 #include "tools/gn/err.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/input_conversion.h"
 #include "tools/gn/input_file.h"
@@ skipping 35 matching lines @@
   if (args.size() != 2) {
     *err = Err(function->function(), "Wrong number of arguments to read_file",
                "I expected two arguments.");
     return Value();
   }
   if (!args[0].VerifyTypeIs(Value::STRING, err))
     return Value();
 
   // Compute the file name.
   const SourceDir& cur_dir = scope->GetSourceDir();
-  SourceFile source_file = cur_dir.ResolveRelativeFile(args[0].string_value());
+  SourceFile source_file = cur_dir.ResolveRelativeFile(args[0].string_value(),
+      scope->settings()->build_settings()->root_path());
   base::FilePath file_path =
       scope->settings()->build_settings()->GetFullPath(source_file);
 
   // Ensure that everything is recomputed if the read file changes.
   g_scheduler->AddGenDependency(file_path);
 
   // Read contents.
   std::string file_contents;
   if (!base::ReadFileToString(file_path, &file_contents)) {
     *err = Err(args[0], "Could not read file.",
                "I resolved this to \"" + FilePathToUTF8(file_path) + "\".");
     return Value();
   }
 
   return ConvertInputToValue(scope->settings(), file_contents, function,
                              args[1], err);
 }
 
 }  // namespace functions